Sandra Betzina did a show on sewing for maternity.  Here is the URL for the 
show notes:
http://www.hgtv.com/HGTV/project/0,1158,CRHO_project_11263,00.html

Since some of you can't access the hgtv site, I copied the notes below.
They may be easier to read at the URL.

Maternity Wear

Expecting a baby? Then you will need a new wardrobe. Here are some
great options for maternity clothes and accessories.

Patterns:

Look for high-waisted baby-doll dress patterns. Go for the big shirt
look. Jump suits with deep pleats are comfortable.

Fabric: Look for very drapey fabric like
wool crepe
soft rayon
wool jersey
cotton knits
sand-washed rayon

Avoid fabric with large stripes

Altering regular patterns for maternity wear:

These are a few of the most flattering and easy-to-alter patterns
for maternity wear:

tunic top with large slits
any oversized tops
apron dress
jackets (but don't pick one that will pull open over your belly)
elastic-waistband pants

Altering Pants for Maternity Wear:

1. You will only change the center front of the pattern.  Start 1" up
   from the crotch and begin to draw upward and away from the pants.

2. Add height to the waistband, extending upward to the point of your
   center front extension. The end result is an upward-slanting triangle.

Altering a Top for Maternity Wear:

1. Draw a line beginning from the bottom toward the top of the
   pattern.  When you get to the armhole area, draw across to the
   right.  You should now have an upside down "L."

2. Draw three of these lines in a stair-step fashion, moving them
   toward the waist.

3. Cut each upside down "L." Spread them apart about 1".

4. Add about 1-1/2" to 2" to the hem, tapering on the sides to the
   original hem length.

To Make Deep Pleats and Add to the Ease Line:

To add extra fullness, add an ease line to the outcurve.

To add deep pleats, sew an inverted pleat, which allows the pleat to
lie flat.

Accessories for Maternity Clothes:

1. If you are expanding rapidly, wear shorter-length necklaces. Long
   bead strands will fall to one side on your tummy.

2. Long, knotted scarves make great accent pieces and tend to focus
   attention on the face, rather than the tummy.

3. Choose a solid base garment and jazz it up with a necklace or vest.

Resources:

McCall's pattern
The patterns seen on this show are generally no longer available.
Visit your local fabric store for similar patterns.

McCall's Pattern Company Consumer Service
11 Penn Plaza
New York, NY 10001 USA
Toll-Free: 800-782-0323
E-mail: consumerservice@mccallpattern.com
Web site: www.mccall.com

Simplicity pattern
The patterns seen on this show are generally no longer available.
Visit your local fabric store for similar patterns.

Simplicity Pattern Company
Consumer Relations
901 Wayne St.
Niles, MI 49121 USA
Toll-Free: 888-588-2700
E-mail: info@simplicity.com
Web site: www.simplicity.com

Maternity wear fabrics

Hancock Fabrics
PO Box 2400
Tupelo, MS 38803-2400 USA
Phone: 662-842-2834
Fax: 662-842-2834
E-mail: consumerdiv@hancockfabrics.com
Web site: www.hancockfabrics.com